Physicochemical analysis of permeability changes in the presence of zinc

Geoderma 145 (2008) 1–7

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Geoderma j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / g e o d e r m a

Review

Physicochemical analysis of permeability changes in the presence of zinc H. Souli a,⁎, J.-M. Fleureau a,1, M. Trabelsi Ayadi b,2, M. Besnard c,3 a b c

Laboratoire de Mécanique, CNRS UMR 8579, Ecole Centrale Paris, Grande voie des vignes, 92295 Châtenay-Malabry, France Laboratoire d'Application de la Chimie aux Ressources et Substances Naturelles et à l'Environnement, Faculté des Sciences de Bizerte, Tunisia Laboratoire de Physico-Chimie, Pharmacotechnie et Biopharmacie, UMR CNRS 8612, Université Paris-Sud 11, Faculté de Pharmacie, IFR 141, 92296 Châtenay-Malabry, France

a r t i c l e

i n f o

Article history: Received 11 July 2007 Received in revised form 17 January 2008 Accepted 26 February 2008 Available online 16 April 2008 Keywords: Clay Microfabric Permeability Pollutant BET MIP XRD SEM

a b s t r a c t Permeability measurements were carried out on specimens of a compacted natural montmorillonite polluted by zinc. The permeability of the polluted clay is higher than that of the clay saturated with water and increases with the zinc concentration between 0.01 and 1 M. The permeability of the sample saturated with water is 2.9 · 10− 12 ms− 1; its value is multiplied by 1.3 when the sample is saturated by a 0.01 M zinc solution and 1.6 for a 1 M solution. The aim of this paper is to study the changes in the particle organisation and porosity of the samples due to the presence of the metals by using several physicochemical methods and to correlate the permeability changes to fabric changes. Both X-ray diffraction and BET measurements indicate an increase in the particle size when the zinc concentration increases from 0 to 1 M. X-ray diffraction results suggest that, for the sample saturated with the 0.01 M solution, the fabric is flocculated (isotropic) whereas, when the zinc concentration increases to 1 M, the fabric becomes more anisotropic. Mercury intrusion porosimetry shows that the increase in the zinc concentration results in an increase in the inter-aggregate pore spaces that become progressively larger, whereas the interparticular pore volume decreases. All these results are supported by SEM observations and highlight the fact that the permeability changes are due to a reorganisation of the particles leading to fabric changes. © 2008 Elsevier B.V. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . Material and methods . . . . . . . . . . . . . . . 2.1. Geotechnical and physicochemical methods . . 2.2. Permeability measurements . . . . . . . . . 2.2.1. Sample preparation . . . . . . . . . 2.2.2. Pulse test. . . . . . . . . . . . . . 3. Experimental results . . . . . . . . . . . . . . . . 3.1. Physicochemical and geotechnical identification 3.2. Void ratio and permeability measurements . . 3.3. Changes in the microfabric of the soil . . . . 3.3.1. Mercury intrusion tests . . . . . . . 3.3.2. X-ray diffraction patterns . . . . . . 3.3.3. BET measurements . . . . . . . . . 3.3.4. SEM observations . . . . . . . . . . 4. Discussion . . . . . . . . . . . . . . . . . . . . . 5. Conclusion. . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . .

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⁎ Corresponding author. Tel.: +331 4113 1706; fax: +331 4113 1442. E-mail addresses: [email protected] (H. Souli), jean-marie.fl[email protected] (J.-M. Fleureau), [email protected] (M. Trabelsi Ayadi), [email protected] (M. Besnard). 1 Tel.: +331 4113 1320; fax: +331 4113 1442. 2 Tel.: +216 71 770 285; fax: +216 71 772 255. 3 Tel.: +331 46 83 5819; fax: +331 46 619 334. 0016-7061/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2008.02.014

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H. Souli et al. / Geoderma 145 (2008) 1–7

1. Introduction

2. Material and methods

Compacted clays are widely used as barriers in industrial waste landfills. There are many kinds of pollutants in the wastes and it is necessary to understand the interactions between clay and pollutants to be able to predict the long-term behaviour of the barrier. The present study focuses on the effect of zinc on the permeability and fabric of a natural montmorillonite. A review of the literature indicates that most of the research carried out in recent years is devoted either to the study of the mechanisms of adsorption of metals on clay or to the macroscopic changes in the hydromechanical behaviour of the material. For instance, Auboiroux et al. (1995), Abollino et al. (2003), Hyun et al. (2000), Brigatti et al. (1995) and Morton et al. (2001) show that the adsorption of heavy metals is strongly influenced by the pH as well as the clay and metal concentrations. Concerning the hydromechanical behaviour of clay in the presence of heavy metals, Jullien et al. (2002) showed that the permeability of an interstratified illite–smectite increased from 1.1 · 10− 12 ms− 1 for samples saturated with water to 2.4 · 10− 12 ms− 1 for samples saturated with copper nitrate. They showed that, when the copper concentration was progressively increased, the permeability increased up to a maximum value after which it remained constant. Souli (2006) showed that the effect of metals on the permeability of a clayey soil depended on its mineralogy: under alkaline pH conditions, the permeability of a carbonated clay sample saturated by a solution containing heavy metals was lower than that of a sample saturated by water. The author attributed the result to the interactions between the metals and the non-swelling minerals, leading to the formation of carbonate precipitates. A few studies tried to establish a relation between the changes in the microfabric of the clay and the presence of heavy metals. For instance, the results obtained by Jozja et al. (2003) showed that saturating a clay sample with Pb2+ ions led to an increase in its permeability from 1.9 · 10− 11 ms− 1 to 7.3 · 10− 10 ms− 1. Scanning electron microscopy and X-ray diffraction analysis revealed that the increase in the permeability value was due to fabric changes either at the nanometric scale (reduction of particle size) or at the micrometric scale (microfissuration of aggregates). Similar results were presented by Shackelford et al. (2000), who showed that the breakdown of the aggregates was necessary to produce an increase in permeability. Egloffstein (2001) also reports examples of ion exchange: replacing sodium by calcium in a bentonite resulted in a change in the fabric of the soil from smaller finely distributed clay flakes to larger crystals. The latter structure led to higher permeability. The role played by fabric changes in permeability changes is confirmed by other researchers (Mansur and Kaufman, 1962; Shackelford, 1993; Schmitz, 2006). In this paper, the results of permeability measurements in a very plastic natural compacted clay are reported. The material was chosen because it appeared well suited to the fabrication of landfill liners. The conclusion of the literature review is that the changes in the macroscopic properties of the soil are seldom related to the microfabric changes that can be derived from a multiscale study. One of the objectives of the paper is therefore to relate the permeability changes to changes in the microfabric of the soil, using different techniques like X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), BET tests and scanning electron microscopy (SEM). The results of these tests will provide information about the fabric changes due to the sample preparation process in the presence of the heavy metals. This process includes compaction of the sample, free swelling and consolidation. The effect of these different solicitations on the fabric changes was studied by Guillot et al. (2002), Cuisinier and Laloui (2004), and Souli et al. (2007). In this paper, the attention is focused on the study of the pore size distribution, the particle size and their orientation in the final state of the sample in which the permeability measurements are performed.

2.1. Geotechnical and physicochemical methods The experiments were carried out on a natural calcic smectite from Milos Island in the Aegean Sea in Greece. The plasticity and sedimentation tests limits were carried out according to the AFNOR standards NFP 94-051 (AFNOR, 1993) and NFP 94-057 (AFNOR, 1992), respectively. Identification of the material is based on mineralogical and oxide composition, cation exchange capacity and specific surface area measurements. The mineralogical composition was determined using a Siemens D5000 diffractometer equipped with an X-ray generator, under a tension of 40 kV and an intensity of 20 mA, with Ni-filtered Cu– Kα (λ = 0.15418 nm); the X-ray diffractograms were recorded for angles ranging from 2.8° to 70°, by steps of 0.02°. The Si, Al, Fe, Mg, K, Na, Mn, Ca and Ti contents were obtained by means of a Jobin-Yvon JY emission spectrophotometer. The value of the cation exchange capacity (CEC) was determined using copper ethylene diamine (Bergaya and Vayer, 1997). The specific surface area (SSa) was measured using the ethylene glycol method (Quirk and Murray, 1999). After the permeability tests, the main properties studied are the void ratio of the samples, the pore size distribution, the evolution of the distance and orientation of the layers using XRD and the specific surface area using the BET method. The total void ratio of the specimens was derived from their dry weight and external volume, measured at the end of the tests; the latter was obtained by weighing the specimen in the air and in a non-wetting liquid (kerdane) of known specific weight. Mercury intrusion tests were performed on freeze-dried clay specimens, about 1 g in mass, using a Micromeritics Auto IV pore 9500 porosimeter in a range of pressures varying from 0.0035 to 200 MPa corresponding, for cylindrical pores, to diameters between 350 and 0.0063 μm. Samples were also observed by SEM using a PHILIPS (XL 30) microscope. The freeze drying protocol described by Tessier and Berrier (1979) was used to dry the specimens for SEM observations and MIP tests. According to the authors, this method, in which the specimen is cooled very quickly to − 40 °C and the ice is eliminated by sublimation is one of those that causes the smallest disturbance in the fabric of the samples during drying. BET tests were performed at liquid–nitrogen temperature (77 K), using a Coulter SA 3100 apparatus. Prior to the determination of an adsorption isotherm, the sample (of approximately 30 mg) was outgassed at 140 °C during 16 h under a residual pressure lower than 10− 2 Pa. The pH of the samples in the presence of water and zinc solutions was measured according to the standard AFNOR procedure NF ISO 10390 (AFNOR, 2002). 2.2. Permeability measurements 2.2.1. Sample preparation For the permeability measurements the samples were prepared in three steps: First step: The samples were compacted to the standard Proctor optimum water content and maximum dry density. The optimum moisture content and maximum density in the presence of water, determined according to the ASTM standard D698-91 (A.S.T.M., 1998), are 40% and 1.11 mg/m3, respectively. Due to the difficulty of obtaining homogenous specimens using the standard dynamic procedure, compaction was carried out in an oedometer under quasi-static conditions by means of a loading frame at the rate of 1 mm/min. After compaction, the dimensions of the specimen were 40 mm in diameter and 20 mm in height. Second step: The samples were hydrated in the compaction cell under the piston weight (10 kPa). The vertical deformation of the samples was recorded versus time and the samples were considered as saturated when the displacement of the piston became constant (less than 0.01 mm in 24 h).

H. Souli et al. / Geoderma 145 (2008) 1–7

3

k is the permeability (ms− 1), Q, the flow of solution (m3 s− 1), i, the hydraulic gradient, dt, the interval of time (s), h, the height of the sample (m), S, the cross-section area of the sample (m2), u0moy, the mean pressure between t and t + dt at the lower end of the sample (kPa), u1, the pressure at the upper end of the sample (kPa). An example of pressure decay in a sample during a pulse test is shown in Fig. 2. Each test was carried out on three identical specimens. A study of the dispersion of the results was performed on all the tests and led to a mean relative deviation of 12%. 3. Experimental results 3.1. Physicochemical and geotechnical identification Fig. 1. Experimental device for permeability measurement.

Third step: The samples were consolidated under an effective confining pressure σ′3 = 200 kPa (i.e. under a total confining stress σ3 = 800 kPa and a back-pressure u = 600 kPa applied at both ends of the sample). Primary consolidation was considered as finished when the volume change of the sample became lower than 3 mm3/24 h. The high value of the effective confining stress was chosen as representative of the stresses applied in-situ on the clay layer at the bottom of a repository, under 20 m of wastes. The 200 kPa stress also corresponds to the value recommended for the determination of the permeability of clay in the Belgian guideline for the construction of sanitary landfills (Marcoen et al., 2000). Three solutions were used to saturate and permeate the specimens: (i) distilled water, (ii) a solution containing Zn2+ ions at a 0.01 M concentration, (iii) a solution containing Zn2+ ions at a 1 M concentration. The solutions were obtained by dissolving the Zn(NO3)2, 6H2O (98%) salt (from Fluka) in deionised water. In all the cases, the same solution was used to hydrate the sample and as permeant in the permeability measurements. This procedure allowed the specimen to reach equilibrium very quickly with the permeant and avoided cation exchanges during the permeability measurements. This procedure with the two zinc concentrations can be considered as corresponding to the study of the longterm behaviour of the barrier at two stages of pollution. 2.2.2. Pulse test The permeability values were measured using the pulse method (Brace et al., 1968) in a flexible wall permeameter linked to 3 pressure– volume controllers (from GDS Ltd.) (Fig. 1). The method consisted in imposing a pressure increase (from u0 = 600 to u0 = 650 kPa) in the lower chamber connected to the base of the sample for 3 min while maintaining the pressure in the upper chamber (u1 = 600 kPa) and the confining pressure constant. Then the volume of the solution in the lower chamber was kept constant and its pressure began to decrease, due to the flow entering the specimen. The pulse value (50 kPa) was chosen as the best compromise between the duration of the test and the disturbance of the specimen. The permeability value was derived from the volume of solution infiltrated under the effect of the hydraulic gradient between the times t and t + dt, which is related to the change in pressure during the same period: dVw ¼ Cw Vw du0

The X-ray diffractogram (Fig. 3) shows a (001) reflection at 1.5 nm related to calcic smectite (Brown and Brindley, 1980). The (003) and (005) reflections are located at 0.497 and 0.295 nm respectively. The appearance of the (060) band at 0.149 nm reveals the dioctaedric character of the studied clay sample. The CEC and specific surface area values of the clay fraction are equal to 1100 mmol/kg of dried clay and 600 m2/g, respectively. These results indicate that the clay fraction is mainly composed of montmorillonite. The oxide percentages shown in Table 1 support this conclusion. The pH of clay suspensions in zinc solutions at the concentrations of 0.01 M and 1 M is equal to 6 and 4.5, respectively. The physical and geotechnical properties of the soil are shown in Table 2. The clay fraction derived from sedimentation tests is approximately 90%. The Atterberg limits show that the sample is very plastic with a liquid limit of 170% and a plastic limit of 60%. 3.2. Void ratio and permeability measurements During the different steps of the preparation process, changes in the void ratio of the specimens are observed. The corresponding void ratio values are indicated in Table 3. All the samples are compacted to the same dry density of 1.45. After the free swell phase, the void ratios increase to 2.10, 2.01 and 1.80 for water, 0.01 M and 1 M zinc solutions, respectively, which is consistent with the widening of the macropores occurring during that phase (Al-Mukhtar et al., 1996; MontesHernandez et al., 2006; Souli et al., 2007). After consolidation, the void ratios decrease to 1.65, 1.60 and 1.50, respectively. These results show that the presence of the metals results in a marked reduction

ð1Þ

where dVw is the volume of water flowing into the specimen under the pressure gradient, Vw, the initial volume of the lower chamber (m3), du0, the pressure change in the lower chamber between t and t + dt (kPa), Cw, the compressibility coefficient of the lower chamber (kPa− 1). The derivation of the permeability k is based on Darcy's law [2]: k¼

Q 1 dVw Cw Vw gw hdu0
¼ ¼ i S iS dt Sdt u0moy  u1 þ gw h

ð2Þ Fig. 2. X-ray diffractogram of the soil sample.

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H. Souli et al. / Geoderma 145 (2008) 1–7 Table 2 Physical and geotechnical properties Physical properties

Geotechnical properties

Clay fraction (b2 μm)

Cation exchange capacity

Specific surface area

Liquid limit

Plastic limit

Plasticity index

% 90

(mmol/kg) 1100 (±5%)

(m2/g) 600 (±5%)

% 170 (±1%)

% 60 (± 3%)

% 110 (±3%)

Table 3 Void ratio values after each step of the preparation process Permeant

Water

Zinc solution at 0.01 M concentration

Zinc solution at 1 M concentration

Void ratio after compaction Void ratio after free swelling Void ratio after consolidation

1.45 (± 0.02)

1.45 (±0.02)

1.45 (±0.02)

2.10 (± 0.02)

2.01 (±0.02)

1.80 (±0.02)

1.65 (± 0.02)

1.60 (±0.02)

1.50 (±0.02)

Table 4 Results of permeability measurements Fig. 3. Typical result of a pulse test.

of the swelling capacity of the soil and a slight decrease in its compressibility. The permeability of the polluted clay is higher than that of the clay saturated with water and increases with the zinc concentration between 0.01 and 1 M: its value varies from 2.9 · 10− 12 ms− 1 for the sample saturated with water to 3.7 and 4.5 · 10− 12 ms− 1 for the samples saturated with zinc at the concentrations of 0.01 and 1 M, respectively (Table 4). It is clear that changes in the fabric of the samples occur during compaction, during free swell and during consolidation. However, it was chosen in this paper to consider only the final state of the specimens and their microfabric in that state, which is the relevant parameter to interpret the permeabilities measured under the same condition. 3.3. Changes in the microfabric of the soil 3.3.1. Mercury intrusion tests The results of the mercury intrusion tests carried out on the samples saturated with water and zinc solutions are shown in Table 5. The changes in the cumulative and incremental volumes versus the pore size diameter are presented in Fig. 4. Comparing the sample saturated with water with that saturated with the 1 M zinc solution shows a decrease in the cumulative volume from 0.43 ml/g to 0.27 ml/ g. Using these results, the ratio of the cumulative volume to the total pore volume derived from the void ratio measurements was calculated. For the sample saturated with water, this ratio is equal to 43% whereas, for the samples saturated with the zinc solutions, the ratio increases to 55% in the case of the 0.01 M concentration and 65% in the case of the 1 M concentration. This increase suggests a change in the porosity of the sample, from closed, inaccessible pores to a more open fabric. In order to explain which pores are influenced by the presence of zinc, the interpretation of the mercury intrusion results is made by considering two families of pores: inter-aggregate pores whose

Permeant

Water

Zinc solution at 0.01 M concentration

Zinc solution at 1 M concentration

Permeability (m/s)

2.9 · 10− 12 (±12%)

3.7 · 10− 12 (± 12%)

4.5 · 10− 12 (±12%)

diameter is larger than 1.5 μm and interparticular pores corresponding to a diameter smaller than 1.5 μm (Tessier et al., 1992; Sala and Tessier, 1993). The results highlight the increase in the inter-aggregate pore volume when the zinc concentration in the solution increases: the inter-aggregate pore volume increases from 0.053 ml/g (5% of the total pore volume) in the case of the sample saturated with water to 0.200 ml/g (48% of the total pore volume) for the sample saturated with the 1 M zinc solution. At the same time, a decrease in the interparticular pore volume is observed. The interparticular pore volume decreases from 0.371 ml/g in the case of the sample saturated with water to 0.0721 ml/g for the 1 M zinc solution. The incremental volume curves show that that the samples saturated with water and with the smallest zinc concentration (0.01 M) present a predominant pore diameter at 0.5 μm. In the case of the sample saturated with zinc at the 1 M concentration, the predominant pore diameter shifts to a higher diameter, around 17 μm. These results highlight the fact that a reorganisation of the particles occurs in the presence of zinc, and differs for each concentration. 3.3.2. X-ray diffraction patterns The evolution of the (001) reflection as a function of the zinc concentration is shown in Fig. 5. The saturation of the sample with the zinc solutions leads to larger interlayer spaces compared to those of the sample saturated with water. The (001) reflection increases from 1.80 nm for the sample saturated with water to 1.93 and 1.90 for the samples saturated with 0.01 and 1 M zinc solutions, respectively. These different values (1.80, 1.90 and 1.93 nm) all correspond to 3 water layers, which indicate a reorganisation of the water molecules in the interlayer spacing (Guillot et al., 2001). For the sample saturated with the 0.01 M solution, the appearance of a reflection at 1.60 nm suggests the existence of two levels of organisation in the sample.

Table 1 Oxide contents SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

MnO

K2O

TiO2

L.O.I.

Total

% 57.65 (b1%)

% 19.32 (b 1%)

% 1.70 (b 5%)

% 1.12 (b 10%)

% 4.48 (b 1%)

% 0.60 (b 10%)

% 0.01 (b10%)

% 0.28 (b 10%)

% 0.16 (b20%)

% 12.38 (b 2%)

% 97.7

H. Souli et al. / Geoderma 145 (2008) 1–7

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Table 5 Cumulative volumes measured by mercury intrusion tests after permeability tests Permeant MIP total intrusion volume (ml/g) Inter-aggregate pore volume (ml/g) (pores N 1.5 μm) Interparticular pore volume (ml/g) (pores b 1.5 μm)

Water

Zinc solution at 0.01 M concentration

Zinc solution at 1 M concentration

0.42 (±0.01)

0.36 (±0.01)

0.27 (±0.01)

0.053 (±0.01)

0.14 (±0.01)

0.20 (±0.01)

0.37 (±0.01)

0.23 (±0.01)

0.072 (±0.01)

On the other hand, the comparison of the full widths at half maximum (FWHM) shows that the samples saturated with water and with the zinc solution at the concentration of 0.01 M present similar values, whereas the FWHM is smaller for the sample saturated with the 1 M zinc solution, suggesting an increase in the number of layers

Fig. 5. Evolution of the (001) reflection of the clay in the presence of water and zinc solutions at the concentrations of 0.01 and 1 M.

per particle (Ducloux et al., 2002). The increase in the intensity of the (001) reflection for the sample saturated with the 1 M solution, compared to the sample saturated with the 0.01 M solution, is a qualitative indicator of a higher degree of organisation of the particles in the presence of zinc (Jozja et al., 2003). 3.3.3. BET measurements The BET method was used in this work only to follow the evolution of the specific surface area of the particles. The results reported in Table 6 show a decrease in the specific surface area when the concentration in zinc increases: the specific surface area of the sample in the presence of water is equal to 9 m2/g; it slightly decreases to 7.9 m2/g in the case of the 0.01 M zinc solution whereas, for the zinc concentration of 1 M, the decrease is much more important (to 5.5 m2/g). These very low values, compared to those usually measured on montmorillonite, are probably due to the formation of aggregates during the preparation process. The decrease in the specific surface area indicates a decrease in the size of the particles. 3.3.4. SEM observations Fig. 6a, b and c presents pictures of the samples saturated with water and with the zinc solutions. Fig. 6a shows that the sample saturated with water presents a compact fabric. For the sample saturated with zinc at the 0.01 M concentration (Fig. 6b), the SEM picture shows that the sample presents a flocculated structure. In contrast, Fig. 6c shows the formation of aggregates, with a large spacing between them. The figure also shows that the aggregates tend to have the same (horizontal) orientation, i.e. that the fabric is more anisotropic. 4. Discussion The permeability tests carried out on clay samples saturated with water and with zinc solutions at the concentrations of 0.01 and 1 M Table 6 Evolution of the BET specific surface area with zinc concentration Specific surface area m2/g

Fig. 4. (a) Cumulative and (b) incremental volume versus pore diameter for specimens saturated with water and zinc solutions at concentrations of 0.01 M and 1 M.

Water Zinc (0.01 M) Zinc (1 M)

9.0 (±0.2) 7.9 (±0.2) 5.5 (±0.2)

6

H. Souli et al. / Geoderma 145 (2008) 1–7

inaccessible pores (Guillot et al., 2002) contributing to the low permeability of the sample. For the sample saturated with the 1 M zinc solution, the fabric of the sample appears more open, which should lead to a higher permeability. This is confirmed by the detailed analysis of the MIP tests showing that, when the zinc concentration increases from 0 (water) to 1 M, the inter-aggregate pore volume increases with the zinc concentration. At the same time, the interparticular pore volume decreases, which is probably associated with a reorganisation of the particles. The results of XRD and BET tests can be used to explain how the pore size distribution changed. First, it is known that the presence of zinc and other divalent cations leads to the aggregation of the particles. Indeed, the increase in the zinc concentration results in a decrease in the double layer thickness and, as a consequence, in lower repulsive forces. The decrease in pH also contributes to the decrease in the repulsive forces (Mitchel, 1993). The outcome is a thickening of the particles with the aggregation of a larger number of layers (Egloffstein, 2001; Ouhadi et al., 2006). This is confirmed by the decrease in the specific surface area of the specimens derived from BET results, suggesting that the particles become closer to each other as the zinc concentration increases (Morvan et al., 1994; Yong and Warkentin, 1966). But the changes in the pore size distribution do not result only from the aggregation of the particles but also from the changes in the organisation of the material. More information about the organisation of the particles is provided by the X-ray diffractograms, BET results and SEM pictures: (i) when water is replaced by the low concentration (0.01 M) zinc solution, the full width at half maximum (FWHM) of the XRD increases and the intensity of the (001) reflection decreases; this behaviour can be attributed to the formation of a flocculated (isotropic) fabric (Ouhadi and Goodarzi, 2006), usually responsible for a permeability increase. At the same time, the BET results show that the decrease in the specific surface area is small, which indicates that the aggregation of particles does not play a major part in the phenomenon under these conditions. The picture of the porous medium at the 0.01 M zinc concentration is that of randomly orientated, rather small, particles, with large voids between them. This is confirmed by SEM photos showing a flocculated fabric with a beginning of aggregate formation. (ii) when increasing the zinc concentration from 0.01 M to 1 M, the intensity of the (001) reflection increases. This indicates that the sample becomes more anisotropic when saturated with zinc at the 1 M concentration. The decrease in the FWHM and specific surface area confirms the increase in the number of layers per particle when the zinc concentration increases. This increase in the size and orientation of the particles at the 1 M zinc concentration under nearly constant void ratio conditions suggests that the porous medium is mainly made of parallel aggregates of particles, leaving larger spaces between the aggregates. Fig. 6. SEM pictures of the clay (a) saturated with water, (b) saturated with the zinc solution at the concentration of 0.01 M (c) saturated with the zinc solution at the concentration of 1 M.

show an increase in permeability with the zinc concentration. The aim of the physicochemical tests is to relate these changes to the changes in the microfabric of the specimens used for the permeability measurements, due to the presence of zinc. The evolution of the pore size distribution derived from mercury intrusion porosimetry shows an increasing contribution of the porosity accessible by mercury to the total porosity: in the case of the sample saturated with water, mercury invades 43% of the total pore volume; this value increases to 65% for the sample saturated with zinc at the 1 M concentration. This result indicates that, in the case of the sample saturated with water, the porosity is mainly made of closed

The SEM photos support this interpretation, showing two phenomena: the formation of large separate aggregates and a preferential orientation of the particles. The result is the predominance of large pores in the material, leading to shorter flow paths and to an increase in permeability. 5. Conclusion In the presence of zinc, the permeability of a montmorillonite increases with the concentration in zinc, between 0 and 1 M. The void ratio values measured after the permeability measurements tend to decrease slightly, suggesting that the permeability changes mainly result from a reorganisation of the particles. Mercury intrusion porosimetry shows that the size of the interparticular pore decreases and that that of the inter-aggregate

H. Souli et al. / Geoderma 145 (2008) 1–7

pores increases as the zinc concentration increases. The association of BET and X-ray diffraction results shows that increasing the zinc concentration leads to an increase in the particle size due to the compression of the double layer and to a decrease in the repulsive forces. The X-ray diffraction results show that the sample saturated with the zinc solution at the 0.01 M concentration, presents a flocculated (isotropic) fabric, whereas, at the 1 M zinc concentration, the fabric becomes more anisotropic. The SEM observations support the conclusions of the other tests: the SEM photos show the formation of a flocculated fabric in the case of the sample saturated with the 0.01 M solution, with a small widening of the pores compared to the sample saturated with water. At this concentration, the isotropic organisation of the particles is responsible for the permeability increase. On the other hand, in the case of the sample saturated with the 1 M zinc solution, larger and more orientated particles are formed, leaving larger pores between them, while interparticular pores become smaller. The SEM observations also confirm the increase in the anisotropy of the sample in the case of the sample saturated with the 1 M solution. The result of these mechanisms is a shorter flow path and, as a consequence, a higher permeability. References Abollino, O., Aceto, M., Malandrino, M., Sarzanini, C., Mentasti, E., 2003. Adsorption of heavy metals on Na-montmorillonites. Effect of pH and organic substances. Water Res. 37, 1619–1627. AFNOR, 1992. Sols: Reconnaissance et Essais - Analyse granulométrique des sols -Méthode par sédimentation. NF P 94-057. "Association Française de Normalisation" publications, Paris. 17 pp. AFNOR, 1993. Sols: Reconnaissance et Essais - Détermination des limites d'Atterberg Limite de liquidité à la coupelle - Limite de plasticité au rouleau. NF P 94-051. "Association Française de Normalisation" publications, Paris. 15 pp. AFNOR, 2002. Qualité des sols-Détermination du pH. AFNOR NF ISO 10390. Indice de classement, vol. X31-117, pp. 339–347. Al-Mukhtar, M., Belanteur, N., Tessier, D., Vanapalli, S.K., 1996. The fabric of a clay soil under controlled mechanical and hydraulic stress states. Appl. Clay Sci. 11, 99–115. A.S.T.M., 1998. Test Method for Laboratory Compaction Characteristics of Soil Using Standard Effort (600 kNm/m3), D698-91, Vol. 04–08. American Society for Testing and Materials. Auboiroux, M., Baillif, P., Touraya, J.-C., Bergaya, F., 1995. Apport d'analyses XPS pour l'étude à force ionique constante des échanges Ca-Cd et Ca-Pb sur une montmorillonite calcique. C. R. Acad. Sci. Paris, Sci. Terre Planètes 327, 727–730. Bergaya, F., Vayer, M., 1997. CEC of clays. Measurement by adsorption of a copper ethylenediamine complex. Appl. Clay Sci. 12, 275–280. Brace, W.F., Walsh, J.B., Frangos, W.T., 1968. Permeability of granite under high pressure. J. Geol. Res. 73, 2225–2236. Brigatti, M.F., Corradini, F., Franchini, G.C., Mazzoni, S., Medici, L., Poppi, L., 1995. Interaction between montmorillonite and pollutants from industrial wastewaters exchange of Zn2+ and Pb2+ from aqueous solutions. Appl. Clay Sci. 9, 383–395. Brown, G., Brindley, G.W., 1980. X-ray procedures for clay mineral identification. In: Brindley, G.W., Brown, G. (Eds.), Crystal Structures of Clay Minerals and their X-ray Identification. Monograph, vol. 5. Min. Soc., London, pp. 305–360. Cuisinier, O., Laloui, L., 2004. Fabric evolution during hydro-mechanical loading of compacted silt. Int. J. Numer. Anal. Methods Geomech. 28, 483–499. Ducloux, J., Guero, Y., Sardini, P., Decarreau, A., 2002. Xerolysis: a hypothetical process of clay particles weathering under Sahelian climate. Geoderma 105, 93–110.

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